1. Field of the Invention
The present invention relates generally to monitoring systems in sequential program control arrangements, and more particularly, to a monitoring system of the playback type in a sequential program control arrangement for controlling the operation of machinery.
2. Description of the Prior Art
There has been proposed a sequential program control for controlling machinery to advancing a series of actions thereof step by step in accordance with a program provided for determining the order of the actions. Such a sequential program control is generally utilized for causing assembly line facilities or machine tools to operate automatically for manufacturing or processing various kinds of products.
For the purpose of monitoring operations of machinery under the sequential program control, it has been known to have a composite program in which a program for monitoring is incorporated in the program for controlling the operations of the machinery so as to issue a warning when an abnormal operation is detected. In the case where such a composite program is used, however, there are disadvantages in that the composite program becomes very large in scale to include a large number of programmed steps and further it is required to remake both the program for controlling the operations of the machinery and the program for monitoring the operations of the machinery when the machinery is changed.
Apart from the monitoring mentioned above, there has been also proposed a monitoring system of the playback type in which machinery is once preliminarily operated correctly under the sequential program control so that operating states of each component of a control arrangement provided for conducting actually the sequential program control for the machinery are stored in a memory sequentially at every operation step in which a change in the operating states raises. Next actual operating states of each component of the control arrangement provided for conducting actually the sequential program control for the machinery are collated with the stored operating states in the memory respectively in sequence so as to check whether the actual operating states coincide with the stored operating states in the memory or not during a practical operation of the machinery under the sequential program control. A warning is issued when the actual operating state which does not coincide with the corresponding stored operating state in the memory is detected. One example of the monitoring system of the playback type in which the operation of the machinery is considered to be in malfunction when at least two successive actual operating states do not coincide with the corresponding stored operating states in the memory, respectively, and actual operating steps immediately before and after the successive operating steps which do not coincide with the corresponding stored operating states in the memory are stored in the memory so as to facilitate to clarify a cause of a malfunction of the machinery, is disclosed in the Japanese patent application published before examination under publication No. 60-238906.
Such a monitoring system of the playback type as described above may encounters problems as mentioned below.
In the case where a couple of switches which are independently in an ON or OFF state are provided in the monitoring system of the playback type to detect actual operating states of components of a control arrangement for conducting actually the sequential program control for machinery and the control arrangement is operative to cause the machinery to have an action by means of operating an actuator, such as a cylinder device, when both the switches are in the ON state to satisfy an AND condition, there are three different transitions of the operating states of the components of the control arrangement to reaching a situation wherein the switches satisfy the AND condition, that is, two transitions of the operating states in each of which one of the switches comes to be in the ON state first and then the other of the switches comes to be the ON state, and another transition in which both switches come to be in the ON state simultaneously. However, only one of three transitions is usually memorized when the operating states of each component of the control arrangement are previously stored in the memory sequentially at every operation steps, and therefore each of other two transitions are dealt with as an incorrect transition regardless of a proper actual operation of the machinery when the actual operating states of each component of the control arrangement are collated with the stored operating states in the memory respectively in sequence.
To avoid this disadvantage, it is considered to store in the memory all transitions of the operating states of each component of the control arrangement possible to appear during the proper actual operation of the machinery. In such a case, however, the memory is required to have an impracticably large storage capacity.
Further, it is also considered to modify the monitoring system of the playback type to perform monitoring only at selected operation steps in each of which a change in action of the machinery caused by an actuator included in the control arrangement arises. In the case of the monitoring system thus modified, however, the results of monitoring are reduced in reliability and further it is difficult to clarify a cause of a malfunction of the machinery when the malfunction is detected as a result of monitoring.